1. Field of the Invention
The invention is directed to a control circuit which in conjunction with a heating coil provides precise, automatic temperature regulation. More particularly, both the power for heating the coil and the power for the determination of the temperature thereof as reflected by the resistance of the coil can be simultaneously provided through the same two wire leads of the coil. Specifically, the temperature of the heater is established by continuous excitation and sensing that is independent of the actuation of the source of heating power to the coil.
2. Description of the Prior Art
Several approaches to the control of temperature for a heater have employed a two wire heater regulator method. Typically the heating coil has a positive temperature coefficient of resistance and the voltage derived from passing a current through the heating coil is compared to a voltage developed across a reference resistor. The difference, if any, between these two voltages is determined and either an increase or a decrease in the level of power delivered to the heating coil is effected. The power level adjustment is reflected in a corresponding increase or decrease in the temperature of the heating coil.
A convenient method for obtaining the voltage difference measurement is to configure the heating coil as one leg of a four leg impedance bridge circuit. Any variance from the null balance condition at the center of the bridge indicates that a power adjustment is required. In a bridge circuit configuration, a first voltage source excites both the heating coil and the reference resistors for temperature sensing and resistance balancing functions and a second voltage source provides power only across the heating coil for temperature adjustment thereof. Bridge excitation for sensing and heater coil power for temperature adjustment are provided across the same two heater coil leads.
The fundamental drawback experienced in this arrangement is centered in the practical need to isolate the low level control voltage for sensing excitation from the AC line voltage for temperature adjustment, because both voltages are simultaneously required to implement their respective functions.
It has been suggested that the heating coil power source be modified to function as a low power excitation source. For example, a heating coil power source can be modified to provide either low level continuous or periodically pulsed or synchronously pulsed sensing voltage. However, either power stage heat dissipation, closed loop regulation, or noise generation must be compromised in order for the heating coil power source to provide the voltage necessary for both the heating and sensing functions.
Other proposed solutions to the dual power requirements of a two wire heating coil include alternating, either periodically and/or synchronously, the excitation source with the power source, or providing a source of continuous AC excitation with a DC power source for heater coil temperature adjustment. In the former proposed solution, the alternate switching of the excitation and power sources as well as their time separation is mandatory, while in the latter, bridge capacitors and a more complex power stage are required.
A further drawback inherent in the prior art of two wire heater control circuits is the impracticability of adequate fusing for overall circuit protection because the initial current is often much higher than the final current at setpoint. Another problem rests in the fact that a component failure in the circuitry controlling the alternation of the excitation source with the power source will most probably result in the cataclysmic failure of the entire heater control circuit.
It is therefore, an object of this invention to provide a two wire heater regulator control circuit which overcomes the aforementioned prior art limitations. The present invention utilizes a continuous sensing voltage and a heater coil power voltage which can be simultaneously applied to the heater coil.
It is also an object of this invention to provide means for the isolation of the sources of sensing voltage and heater coil voltage.
It is a further object of this invention to provide a short circuit protection circuit responsive to heater coil failure.